1. Field of the Invention
This invention relates to a hitless switching apparatus and method for optical transmission lines in an optical network.
2. Description of the Related Art
In an optical network which includes a plurality of nodes and optical transmission lines formed from optical fibers or the like for interconnecting the nodes, it is sometimes desired to switch a route along which a signal is transmitted without interrupting transmission of a signal between two nodes. Switching is desired, for example, when it is tried, after the signal transmission route is switched to a bypass route due to the occurrence of trouble in a certain optical transmission line or node, to switch the signal transmission line back to the original route after recovery from the trouble, or when another route must be used to transmit a signal in order to perform construction or maintenance of the equipment. Here, to switch the signal transmission route to a different optical fiber accommodated in the same optical fiber cable is included within switching the signal transmission route to another route. Since switching of an optical transmission generally takes place independently of communication between users in a network, a countermeasure must be taken upon switching to prevent the switching from causing an obstacle to regular communication of the users. In particular, hitless switching must be achieved so that an error of even one bit is not produced. In this instance, since a transmission route before switching and another transmission route after switching generally present different propagation delay amounts, switching that absorbs the difference between the propagation delay amounts must be realized. Since the difference in propagation delay time between the routes is quite large compared with the time per signal bit, mere switching between transmission lines using an optical switch cannot realize hitless switching. Even for switching to a different optical fiber in the same optical fiber cable, since the transmission rate is as high as 1 Gbit/second or more, the phase of the signal cannot be identical between the optical fibers and it is necessary to compensate for the difference in propagation delay time.
A hitless switching system suitable for use with an optical network is disclosed, for example, in Japanese Patent Laid-Open Application No. 80636/91 (JP, A, 3-80636) or No. 56925/92 (JP, A, 4-56925). FIG. 1 shows the construction of the hitless switching system disclosed in the former application. An optical fiber cable 103 accommodates n optical fibers 104.sub.a to 104.sub.n for signal transmission constituting n/2 pairs of bidirectional transmission lines, and a single spare optical fiber 108. The optical fibers 104.sub.1 to 104.sub.n for signal transmission are removably connected at one end side thereof to output ports of optical dividers 105.sub.1 to 105.sub.n by way of connectors 109, respectively. The input ports of the optical dividers 105.sub.1 to 105.sub.n are connected to optical signal transmitters 101.sub.1 to 101.sub.n by way of linear polarizers 107.sub.1 to 107.sub.n, respectively. Meanwhile, the other ends of the optical fibers 104.sub.1 to 104.sub.n are removably connected to the input ports of optical combiners 106.sub.1 to 106.sub.n by way of connectors 109, respectively. The output ports of the optical combiners 106.sub.1 to 106.sub.n are connected to optical signal receivers 102.sub.1 to 102.sub.n, respectively. The spare optical fiber 108 has a pair of connectors 109 attached to the opposite ends thereof and has a polarization orthogonal rotator 110 and an optical path length adjuster 111 interposed therein. Each of the connectors 109 can be connected arbitrarily to one of the optical dividers 105.sub.1 to 105.sub.n and one of the optical combiners 106.sub.1 to 106.sub.n. Further, an optical path controller 112 which can be connected arbitrarily to two of the optical combiners by way of connectors 109 is provided. The controller 112 measures the polarization directions of and the difference in time between two optical signals inputted to the optical combiners to control the polarization orthogonal rotator 110 and the optical path length adjuster 111.
In order to switch the transmission route from a first optical fiber for signal transmission to a second optical fiber for signal transmission, the spare optical fiber 108 is first connected to the free ports of an optical divider 105.sub.n and an optical combiner 106.sub.n to which the first optical fiber is connected to set a bypass route by the spare optical fiber 108. Then, the optical divider 105.sub.n and the polarization orthogonal rotator 110 are operated so that a signal light beam in the first optical fiber 104.sub.n and another signal light beam in the spare optical fiber 108 may be linearly polarized perpendicularly to each other, and the signal light beams are introduced into both of the optical fibers 104.sub.n and 108. Thereafter, the optical path length adjuster 111 is adjusted so that the signals passing in the two optical fibers 104.sub.n and 108 do not have a difference between arriving times, and then, the optical combiner 106.sub.n is switched to the spare optical fiber 108 side to set the signal transmission route to the spare optical fiber 104.sub.n 108. Then, the first optical fiber is removed from the optical divider 105.sub.n and the optical combiner 106.sub.n ; instead, a second optical fiber is connected to the optical divider 105.sub.n and the optical combiner 106.sub.n. Thereafter, a similar procedure is followed to switch the transmission route for an optical signal from the spare optical fiber 108 to the second optical fiber, thereby realizing hitless switching from the first optical fiber 104.sub.n to a second optical fiber.
Separately, FIG. 2 shows the construction of the hitless switching system disclosed in Japanese Patent Laid-Open Application No. 56925/92. The system is constructed so that an optical transmitter 121 and an optical receiver 123 are interconnected by way of a pair of optical fibers 122 and 125 provided in parallel to each other. While the optical fiber 122 has an ordinary construction, the other optical fiber 125 has an optical path length adjuster 127 and a polarization controller 128 interposed therein. An optical switch 124 of the 1.times.2 construction for inputting signal light at an arbitrary branching ratio into the optical fibers 122 and 125 is provided on the optical transmitter 121 side. An optical coupler 126 for combining signal light inputted thereto from the two optical fibers 122 and 125 is provided on the optical receiver 123 side. Also hitless switching by the system is performed by adjusting the polarization conditions of the two optical fibers 122 and 125 so that they may be orthogonal to each other and by varying the branching ratio of the optical switch 124 while adjusting the propagation time of signal light in the other optical fiber 125 by means of the optical path length adjuster 127.
With the conventional hitless switching systems described above, however, In order to prevent possible interference between signal light beams from two optical transmission lines when the signal light beams are combined, the polarization directions of the two signal light beams to be combined must be adjusted so as to be orthogonal to each other. Accordingly, optical transmission lines by which a polarization condition is maintained must be used; as well, a polarization controller must necessarily be inserted in at least one of the two optical transmission lines. Further, in order to compensate for the propagation delay time, an optical path length adjuster must necessarily be inserted in at least one of the two optical transmission lines. In other words, the conventional hitless switching systems disadvantageous in that a special optical transmission line must be used, which complicates the entire system.